Onboard anemometer measurements notably aim to determine the following values located at infinity upstream: the true airspeed (or TAS), the angle of attack, the sideslip, the calibrated airspeed and the static pressure of the aircraft or the pressure altitude.
It will be recalled that:
the true airspeed V is the speed of an aircraft relative to undisturbed air;
the angle of attack is the angle α formed between the velocity vector V of the aircraft and the longitudinal axis;
the sideslip is the angle between the velocity vector V and the (vertical) plane of symmetry of the aircraft;
the static pressure (Ps) is a pressure measured for example by a static pressure probe;
the total pressure (Pt) is the pressure measured by a pitot tube, for example;
the calibrated airspeed (Vc) is the speed used to determine the stall speed of the aircraft (this is the speed that the aircraft would be moving at, relative to the ground, under standard conditions, with the same measured pressure Pt less Ps); and
the pressure altitude is the altitude corresponding to the static pressure in the Standard Atmosphere Table defined by the International Civil Aviation Organisation.
These values correspond to infinity upstream of the aircraft. Infinity upstream is a distance upstream of the aircraft that is sufficiently large (for example six fuselage diameters of the aircraft) that the movement of the air induced by the movement of said aircraft does not disturb the aerodynamic field.
Current systems determine these values at infinity upstream using local clinometer or anemometer measurements. These local measurements are carried out near the fuselage and are therefore subject to the disturbances generated by the latter.
The disturbances induced by the aircraft may be calculated so as to adequately correct the local measurements. In order to establish these corrections, prior calibration needs to be carried out during test flights undertaken by the manufacturer.
Systems according to the prior art used to determine the airspeed of an aircraft comprise:
an angle-of-attack probe placed in a particular place (local angle of attack insensitive to sideslip) and measuring a local angle of attack αloc;
optionally a sideslip probe measuring a local sideslip βloc;
a pitot tube measuring a total pressure Pt; and
a pair of left/right static-pressure probes placed in particular places (where the average L/R (left/right) static pressure does not depend on the sideslip, and for which a pneumatic average of the L/R pressures is calculated, corrected by SSEC (static source error correction) laws, so as to determine the local static pressure Ps; and
a probe for measuring an impact temperature Ti (or total air temperature TAT) which is the temperature due to kinetic heating during movement of the aircraft.
FIG. 1 illustrates a local angle of attack close to a fuselage of an aircraft. This figure shows a plane 100 tangent to a fuselage 101 (assumed to be cylindrical) of an aircraft. The aircraft has a longitudinal axis 102 and a plane of symmetry 103.
It will be recalled that t local angle of attack αloc is the angle between, on the one hand, a velocity vector Vloc measured locally (i.e. close to the fuselage) in the plane 100 tangent to the fuselage 101 and, on the other hand, a vector u located in the tangent plane and parallel to the longitudinal axis of the aircraft 102.
The probe measuring the local angle of attack αloc is for example a vane (i.e. a small moveable fin) aligning to the wind direction.
Alternatively, the local angle of attack αloc may also be determined indirectly by carrying out two measurements:
measurement of the velocity vector u located in tangent the plane 100 and parallel to the longitudinal axis of the aircraft 102; and
measurement of a velocity vector v located in tangent the plane 100 and orthogonal to the velocity vector u.
The local angle of attack αloc is then determined using these two values by applying the following formula: αloc=tan−1(v/u).
As for the local angle of attack, it is possible to measure the sideslip directly using a vane that aligns to the wind direction.
It is also possible to determine the sideslip using the ratio of two measured components.
FIG. 2 shows an example of a device according to the prior art for determining the angle of attack and the sideslip of an aircraft using local angle-of-attack measurements.
This device comprises a first vane 201 located on one side of the fuselage 101 and a second vane 202 located on another side of the fuselage, symmetrically to the first vane about the plane of symmetry of the aircraft. The first vane 201 measures a first local angle of attack αlocG and the second vane 202 measures a second local angle of attack αlocD. The angle of attack α and the sideslip β at infinity upstream of the aircraft may then be determined using the above local measurements by applying the following relationships:α=f((αlocD+αlocG)/2)β=g((αlocD−αlocG)/2)where f and g are functions that depend on the aerodynamic properties of the aircraft and that take into account corrections related to the disturbances mentioned above.
FIG. 3 shows the way in which airspeed parameters are determined using the local measurements taken.
The calibrated airspeed Vc is derived from the difference ΔP between the total pressure Pt and the local static pressure Ps.
The Mach number M is determined from the ratio of, on the one hand, the difference ΔP between the total pressure Pt and the local static pressure Ps to, on the other hand, the local static pressure Ps.
The static temperature T is determined from the impact temperature Ti and the Mach number M.
The Mach number M and the static temperature T then make it possible to calculate the magnitude of the true airspeed vector V.
The local static pressure Ps and the static temperature T then make it possible to calculate the density e of the air.
The magnitude of the true airspeed vector, the angle of attack and the sideslip angle at infinity upstream of the aircraft allows the true airspeed vector of the aircraft to be completely defined.
These systems notably have the drawback of requiring measurement instruments, for example vanes, which are fragile, because they are light, and particularly sensitive to icing.
These systems are furthermore difficult to install because the measurement instruments must moreover be placed at specific points on the fuselage of the aircraft so as to minimize measurement errors related to disturbances.
A system for measuring the true airspeed of an aircraft using a laser anemometer focused far from the fuselage (at least 100 m away) and carrying out at least three measurements at three different points in space is known already. However, such a system has the drawback of requiring a powerful laser with a wide aperture. In addition, determining three appropriate measurement points can be difficult.